The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

throwing light on the origin of the infertility of many allied species.

This will be better understood after considering the facts which will be

now described.

_The Isolation of Varieties by Selective Association._
In the last chapter I have shown that the importance of geographical

isolation for the formation of new species by natural selection has been

greatly exaggerated, because the very change of conditions, which is

the initial power in starting such new forms, leads also to a local or

stational segregation of the forms acted upon. But there is also a very

powerful cause of isolation in the mental nature--the likes and

dislikes--of animals; and to this is probably due the fact of the

comparative rarity of hybrids in a state of nature. The differently

coloured herds of cattle in the Falkland Islands, each of which keeps

separate, have been already mentioned; and it may be added, that the

mouse-coloured variety seem to have already developed a physiological

peculiarity in breeding a month earlier than the others. Similar facts

occur, however, among our domestic animals and are well known to

breeders. Professor Low, one of the greatest authorities on our

domesticated animals, says: "The female of the dog, when not under

restraint, makes selection of her mate, the mastiff selecting the

mastiff, the terrier the terrier, and so on." And again: "The Merino

sheep and Heath sheep of Scotland, if two flocks are mixed together,

each will breed with its own variety." Mr. Darwin has collected many

facts illustrating this point. One of the chief pigeon-fanciers in

England informed him that, if free to choose, each breed would prefer

pairing with its own kind. Among the wild horses in Paraguay those of

the same colour and size associate together; while in Circassia there

are three races of horses which have received special names, and which,

when living a free life, almost always refuse to mingle and cross, and

will even attack one another. On one of the Faroe Islands, not more than

half a mile in diameter, the half-wild native black sheep do not readily

mix with imported white sheep. In the Forest of Dean, and in the New

Forest, the dark and pale coloured herds of fallow deer have never been

known to mingle; and even the curious Ancon sheep of quite modern origin

have been observed to keep together, separating themselves from the rest

of the flock when put into enclosures with other sheep. The same rule

applies to birds, for Darwin was informed by the Rev. W.D. Fox that his

flocks of white and Chinese geese kept distinct.[61]

slightly different varieties of the same species, is evidently a fact

of great importance in considering the origin of species by natural

selection, since it shows us that, so soon as a slight differentiation

of form or colour has been effected, isolation will at once arise by the

selective association of the animals themselves; and thus the great

stumbling-block of "the swamping effects of intercrossing," which has

been so prominently brought forward by many naturalists, will be

completely obviated.
If now we combine with this fact the correlation of colour with

important constitutional peculiarities, and, in some cases, with

infertility; and consider, further, the curious parallelism that has

been shown to exist between the effects of changed conditions and the

intercrossing of varieties in producing either an increase or a decrease

of fertility, we shall have obtained, at all events, a starting-point

for the production of that infertility which is so characteristic a

feature of distinct species when intercrossed. All we need, now, is some

means of increasing or accumulating this initial tendency; and to a

discussion of this problem we will therefore address ourselves.

_The Influence of Natural Selection upon Sterility and Fertility._
It will occur to many persons that, as the infertility or sterility of

incipient species would be useful to them when occupying the same or

adjacent areas, by neutralising the effects of intercrossing, this

infertility might have been increased by the action of natural

selection; and this will be thought the more probable if we admit, as we

have seen reason to do, that variations in fertility occur, perhaps as

frequently as other variations. Mr. Darwin tells us that, at one time,

this appeared to him probable, but he found the problem to be one of

extreme complexity; and he was also influenced against the view by many

considerations which seemed to render such an origin of the sterility or

infertility of species when intercrossed very improbable. The fact that

species which occupy distinct areas, and which nowhere come in contact

with each other, are often sterile when crossed, is one of the

difficulties; but this may perhaps be overcome by the consideration

that, though now isolated, they may, and often must, have been in

contact at their origination. More important is the objection that

natural selection could not possibly have produced the difference that

often occurs between reciprocal crosses, one of these being sometimes

fertile, while the other is sterile. The extremely different amounts of

infertility or sterility between different species of the same genus,

the infertility often bearing no proportion to the difference between

the species crossed, is also an important objection. But none of these

objections would have much weight if it could be clearly shown that

natural selection _is_ able to increase the infertility variations of

incipient species, as it is certainly able to increase and develop all

useful variations of form, structure, instincts, or habits. Ample causes

of infertility have been shown to exist, in the nature of the organism

and the laws of correlation; the agency of natural selection is only

needed to accumulate the effects produced by these causes, and to render

their final results more uniform and more in accordance with the facts

that exist.
About twenty years ago I had much correspondence and discussion with Mr.

Darwin on this question. I then believed that I was able to demonstrate

the action of natural selection in accumulating infertility; but I could

not convince him, owing to the extreme complexity of the process under

the conditions which he thought most probable. I have recently returned

to the question; and, with the fuller knowledge of the facts of

variation we now possess, I think it may be shown that natural selection

_is_, in some probable cases at all events, able to accumulate

variations in infertility between incipient species.
The simplest case to consider, will be that in which two forms or

varieties of a species, occupying an extensive area, are in process of

adaptation to somewhat different modes of life within the same area. If

these two forms freely intercross with each other, and produce mongrel

offspring which are quite fertile _inter se_, then the further

differentiation of the forms into two distinct species will be retarded,

or perhaps entirely prevented; for the offspring of the crossed unions

will be, perhaps, more vigorous on account of the cross, although less

perfectly adapted to the conditions of existence than either of the pure

breeds; and this would certainly establish a powerful antagonistic

influence to the further differentiation of the two forms.
Now, let us suppose that a partial sterility of the hybrids between the

two forms arises, in correlation with the different modes of life and

the slight external or internal peculiarities that exist between them,

both of which we have seen to be real causes of infertility. The result

will be that, even if the hybrids between the two forms are still freely

produced, these hybrids will not themselves increase so rapidly as the

two pure forms; and as these latter are, by the terms of the problem,

better suited to their conditions of life than are the hybrids between

them, they will not only increase more rapidly, but will also tend to

supplant the hybrids altogether whenever the struggle for existence

becomes exceptionally severe. Thus, the more complete the sterility of

the hybrids the more rapidly will they die out and leave the two parent

forms pure. Hence it will follow that, if there is greater infertility

between the two forms in one part of the area than the other, these

forms will be kept more pure wherever this greater infertility prevails,

will therefore have an advantage at each recurring period of severe

struggle for existence, and will thus ultimately supplant the less

infertile or completely fertile forms that may exist in other portions

of the area. It thus appears that, in such a case as here supposed,

natural selection would preserve those portions of the two breeds which

were most infertile with each other, or whose hybrid offspring were most

infertile; and would, therefore, if variations in fertility continued to

arise, tend to increase that infertility. It must particularly be noted

that this effect would result, not by the preservation of the infertile

variations on account of their infertility, but by the inferiority of

the hybrid offspring, both as being fewer in numbers, less able to

continue their race, and less adapted to the conditions of existence

than either of the pure forms. It is this inferiority of the hybrid

offspring that is the essential point; and as the number of these

hybrids will be permanently less where the infertility is greatest,

therefore those portions of the two forms in which infertility is

greatest will have the advantage, and will ultimately survive in the

struggle for existence.
The differentiation of the two forms into distinct species, with the

increase of infertility between them, would be greatly assisted by two

other important factors in the problem. It has already been shown that,

with each modification of form and habits, and especially with

modifications of colour, there arises a disinclination of the two forms

to pair together; and this would produce an amount of isolation which

would greatly assist the specialisation of the forms in adaptation to

their different conditions of life. Again, evidence has been adduced

that change of conditions or of mode of life is a potent cause of

disturbance of the reproductive system, and, consequently, of

infertility. We may therefore assume that, as the two forms adopted more

and more different modes of life, and perhaps acquired also decided

peculiarities of form and coloration, the infertility between them would

increase or become more general; and as we have seen that every such

increase of infertility would give that portion of the species in which

it arose an advantage over the remaining portions in which the two

varieties were more fertile together, all this induced infertility would

maintain itself, and still further increase the general infertility

between the two forms of the species.
It follows, then, that specialisation to separate conditions of life,

differentiation of external characters, disinclination to cross-unions,

and the infertility of the hybrid produce of these unions, would all

proceed _pari passu_, and would ultimately lead to the production of two

distinct forms having all the characteristics, physiological as well as

structural, of true species.

general infertility might arise in correlation with the different modes

of life of two varieties or incipient species. A considerable body of

facts already adduced renders it probable that this _is_ the mode in

which any widespread infertility would arise; and, if so, it has been

shown that, by the influence of natural selection and the known laws

which affect varieties, the infertility would be gradually increased.

But, if we suppose the infertility to arise sporadically within the two

forms, and to affect only a small proportion of the individuals in any

area, it will be difficult, if not impossible, to show that such

infertility would have any tendency to increase, or would produce any

but a prejudicial effect. If, for example, five per cent of each form

thus varied so as to be infertile with the other form, the result would

be hardly perceptible, because the individuals which formed cross-unions

and produced hybrids would constitute a very small portion of the whole

species; and the hybrid offspring, being at a disadvantage in the

struggle for existence and being themselves infertile, would soon die

out, while the much more numerous fertile portion of the two forms would

increase rapidly, and furnish a sufficient number of pure-bred offspring

of each form to take the place of the somewhat inferior hybrids between

them whenever the struggle for existence became severe. We must suppose

that the normal fertile forms would transmit their fertility to their

progeny, and the few infertile forms their infertility; but the latter

would necessarily lose half their proper increase by the sterility of

their hybrid offspring whenever they crossed with the other form, and

when they bred with their own form the tendency to sterility would die

out except in the very minute proportion of the five per cent

(one-twentieth) that chance would lead to pair together. Under these

circumstances the incipient sterility between the two forms would

rapidly be eliminated, and could never rise much above the numbers which

were produced by sporadic variation each year.
It was, probably, by a consideration of some such case as this that Mr.

Darwin came to the conclusion that infertility arising between incipient

species could not be increased by natural selection; and this is the

more likely, as he was always disposed to minimise both the frequency

and the amount even of structural variations.
We have yet to notice another mode of action of natural selection in

favouring and perpetuating any infertility that may arise between two

incipient species. If several distinct species are undergoing

modification at the same time and in the same area, to adapt them to

some new conditions that have arisen there, then any species in which

the structural or colour differences that have arisen between it and its

varieties or close allies were correlated with infertility of the

crosses between them, would have an advantage over the corresponding

varieties of other species in which there was no such physiological

peculiarity. Thus, incipient species which were infertile together would

have an advantage over other incipient species which were fertile, and,

whenever the struggle for existence became severe, would prevail over

them and take their place. Such infertility, being correlated with

constitutional or structural differences, would probably, as already

suggested, go on increasing as these differences increased; and thus, by

the time the new species became fully differentiated from its parent

form (or brother variety) the infertility might have become as well

marked as we usually find it to be between distinct species.

importance as bearing on the difficult problem of the cause of the

sterility of the hybrids between distinct species. Accepting, as highly

probable, the fact of variations in fertility occurring in correlation

with variations in habits, colour, or structure, we see, that so long as

such variations occurred only sporadically, and affected but a small

proportion of the individuals in any area, the infertility could not be

increased by natural selection, but would tend to die out almost as fast

as it was produced. If, however, it was so closely correlated with

physical variations or diverse modes of life as to affect, even in a

small degree, a considerable proportion of the individuals of the two

forms in definite areas, it would be preserved by natural selection, and

the portion of the varying species thus affected would increase at the

expense of those portions which were more fertile when crossed. Each

further variation towards infertility between the two forms would be

again preserved, and thus the incipient infertility of the hybrid

offspring might be increased till it became so great as almost to amount

to sterility. Yet further, we have seen that if several competing

species in the same area were being simultaneously modified, those

between whose varieties infertility arose would have an advantage over

those whose varieties remained fertile _inter se_, and would ultimately

supplant them.

assumption that some amount of infertility characterises the distinct

varieties which are in process of differentiation into species; and it

may be objected that of such infertility there is no proof. This is

admitted; but it is urged that facts have been adduced which render such

infertility probable, at least in some cases, and this is all that is

required. It is by no means necessary that _all_ varieties should

exhibit incipient infertility, but only, some varieties; for we know

that, of the innumerable varieties that occur but few become developed

into distinct species, and it may be that the absence of infertility, to

obviate the effects of intercrossing, is one of the usual causes of

their failure. All I have attempted to show is, that _when_ incipient

infertility does occur in correlation with other varietal differences,

that infertility can be, and in fact must be, increased by natural

selection; and this, it appears to me, is a decided step in advance in

the solution of the problem.[62]

_Physiological Selection._
Another form of infertility has been suggested by Professor G.J. Romanes

as having aided in bringing about the characteristic infertility or

sterility of hybrids. It is founded on the fact, already noticed, that

certain individuals of some species possess what may be termed selective

sterility--that is, while fertile with some individuals of the species

they are sterile with others, and this altogether independently of any

differences of form, colour, or structure. The phenomenon, in the only

form in which it has been observed, is that of "infertility or absolute

sterility between two individuals, each of which is perfectly fertile

with all other individuals;" but Mr. Romanes thinks that "it would not

be nearly so remarkable, or physiologically improbable, that such

incompatibility should run through a whole race or strain."[63]

Admitting that this may be so, though we have at present no evidence

whatever in support of it, it remains to be considered whether such

physiological varieties could maintain themselves, or whether, as in the

cases of sporadic infertility already discussed, they would necessarily

die out unless correlated with useful characters. Mr. Romanes thinks

that they would persist, and urges that "whenever this one kind of

variation occurs _it cannot escape the preserving agency_ of

physiological selection. Hence, even if it be granted that the variation

which affects the reproductive system in this particular way is a

variation of comparatively rare occurrence, still, as _it must always be

preserved_ whenever it does occur, its influence in the manufacture of

specific types _must be cumulative_." The very positive statements which

I have italicised would lead most readers to believe that the alleged

fact had been demonstrated by a careful working out of the process in

some definite supposed cases. This, however, has nowhere been done in

Mr. Romanes' paper; and as it is _the_ vital theoretical point on which

any possible value of the new theory rests, and as it appears so opposed

to the self-destructive effects of simple infertility, which we have

already demonstrated when it occurs between the intermingled portion of

two varieties, it must be carefully examined. In doing so, I will

suppose that the required variation is not of "rare occurrence," but of

considerable amount, and that it appears afresh each year to about the

same extent, thus giving the theory every possible advantage.
Let us then suppose that a given species consists of 100,000 individuals

of each sex, with only the usual amount of fluctuating external

variability. Let a physiological variation arise, so that 10 per cent of

the whole number--10,000 individuals of each sex--while remaining

fertile _inter se_ become quite sterile with the remaining 90,000. This

peculiarity is not correlated with any external differences of form or

colour, or with inherent peculiarities of likes or dislikes leading to

any choice as to the pairing of the two sets of individuals. We have now

to inquire, What would be the result?
Taking, first, the 10,000 pairs of the physiological or abnormal

variety, we find that each male of these might pair with any one of the

whole 100,000 of the opposite sex. If, therefore, there was nothing to

limit their choice to particular individuals of either variety, the

probabilities are that 9000 of them would pair with the opposite

variety, and only 1000 with their own variety--that is, that 9000 would

form sterile unions, and only _one_ thousand would form fertile unions.
Taking, next, the 90,000 normal individuals of either sex, we find, that

each male of these has also a choice of 100,000 to pair with. The

probabilities are, therefore, that nine-tenths of them--that is,

81,000--would pair with their normal fellows, while 9000 would pair with

the opposite abnormal variety forming the above-mentioned sterile

unions.
Now, as the number of individuals forming a species remains constant,

generally speaking, from year to year, we shall have next year also

100,000 pairs, of which the two physiological varieties will be in the

proportion of eighty-one to one, or 98,780 pairs of the normal variety

to 1220[64] of the abnormal, that being the proportion of the fertile

unions of each. In this year we shall find, by the same rule of

probabilities, that only 15 males of the abnormal variety will pair with

their like and be fertile, the remaining 1205 forming sterile unions